Molded boards of calcium silicate are widely used as building materials, structure materials or the like because these boards are lightweight and outstanding in fire resistance, heat resistance, etc. However, the calcium silicate molded boards as compared with wood are not fully satisfactory in mechanical strength and are low in water resistance. Further these molded boards have the drawbacks of being poor in workability such as suitability for being cut, machinability, nailability, nail-retaining ability and properties of permitting carving and modeling such as suitability for being grooved, and thus unavoidably have a limited use.
In recent years, it has been proposed to incorporate a vinyl-type polymer, natural or synthetic rubber of like polymer into molded boards of calcium silicate in order to eliminate the foregoing drawbacks of the boards (e.g. Japanese Unexamined Patent Publications Nos. 15516/1977; 16024/1979; 264355/85; and 123053/1987). These publications specifically disclose calcium silicate molded boards having a single-layer structure in which the secondary particles of calcium silicate crystals are interconnected with one another directly and via the polymer.
Such molded boards are given enhanced mechanical strength and improved water resistance and workability due to the presence of the resin. Yet the mechanical strength of the boards remains unsatisfactory. With an increase of specific gravity, a calcium silicate molded product is given enhanced strength in proportion but is deprived of its lightweight property among the characteristics of calcium silicate molded product. While the strength of calcium silicate molded product can be improved by using an increased amount of resin, the increase of resin content lowers the incombustibility and heat resistance of the molded product, leading to loss of characteristics of calcium silicate molded product. Because of the single-layer structure, said molded product has high density in the surface portion but low density in the interior, namely irregular density as a whole, so that when exposed to high temperatures as in a fire, the molded product is likely to shrink, deform, warp and crack. Although they have improved workability, such molded product tends to chip off at the cut end during cutting or machining operation and to remain insufficient in nail-retaining ability and carving and modeling properties, such as suitability for being grooved.
Japanese Examined Patent Publication No. 40896/1973 discloses a board of calcium silicate crystals composed of secondary particle of xonotlite crystals and wood fibers. Said board is produced by forming a single layer of wet mat from an aqueous slurry of secondary particles of xonotlite crystals and wood fibers with use of a forming machine and press-molding the layer of wet mat with heating. The board of calcium silicate crystals obtained by such method is one press-molded from the single layer of wet mat and contains as the constituents the secondary particles of xonotlite crystals and wood fibers.
The board of calcium silicate crystals having the above construction is insufficient in the strength, particularly bending strength, even if the board is composed predominantly of wood fibers, that is containing 50% or more thereof. This tendency is pronounced in the board composed predominantly of calcium silicate crystals, more specifically containing a small amount of wood fibers and 50% or more of calcium silicate crystals. In particular the board containing 80% or more of calcium silicate crystals has an unsatisfactory strength. On the other hand, the smaller the amount of wood fibers, the more preferable the board is from an incombustibility view point. However, the aforesaid technique can not produce boards sufficient in both incombustibility and satisfactorily great strength. The '896 publication teaches nothing about the use of inorganic fibers to produce an article of great strength. Among the boards thus produced, those of large thickness tend to have a reduced bending strength due to the irregularity of strength caused by the difference in the proportions of calcium silicate crystals and wood fibers between the internal and external board portions. Further this tendency becomes marked in boards having a thickness of 20 mm or more.